The "daisy chain" type of connection for a plurality of units working under control of a central controlling unit constitute a conventional configuration and method of operation for selecting a unit in the chain for reception, control or transmission of data.
For example, a communication controller which processes the data to and from communication lines comprises a central control unit and several types of adapters such as line adapters, channel adapters, etc. A network control program such as IBM NCP runs in the central control unit and controls the operation of the adapters through either program-initiated operation or adapter-initiated operations. The adapters of each given type, for example channel adapters are connected through a common line in a daisy chain configuration and mode of operation.
The adapters communicate with the central control unit through interrupts. The central control unit services the interrupts raised by the adapters. The selection of each adapter may be done either by the control program through a specific command or by an autoselection mechanism involving hardware. The autoselecton mechanism is started on a broadcast command to the adapters. On this command, the adapters use the daisy chain line to perform the selection of any connected adapter having an interrupt pending.
In such an environment, if an adapter is failing or has to be disconnected from the daisy chain, the daisy chain operation is interrupted.
U.S. Pat. No. 4 380 052 describes a mechanism which allows such a problem to be solved.
According to this patent, each unit Pn, if said unit is not to be selected and has not received the signal to be propagated within a fixed time duration, comprises an alert circuit which is responsive to the arrival of a signal to be propagated in the chain, to the preceding unit P(n-1) to cause a signal to be restarted to the next unit P(n+1).
This mechanism presents the following drawbacks:
It does not operate if more than one unit are failing and have to be bypassed, because when two or three units in series are failing the signal from the previous unit is no longer received.
Furthermore, if a failing unit self-restarts its operation, there may be two signals which are propagated in the chain.